1. Field of the Invention
The invention under consideration relates to a combustor for a gas turbine and, in addition, relates to an associated operating method.
2. Brief Description of the Related Art
U.S. Pat. No. 6,370,863 B2 discloses a combustor for a gas turbine, which has a burner system which has a plurality of burner groups with a plurality of burners in each case. Furthermore, a fuel supply system is provided, which has a main line which is connected to a fuel source, and also, for each burner group, an auxiliary line which is connected to each burner of the associated burner group and connected to the main line by a controllable distribution valve. In addition, a combustion chamber is provided, with the burners being installed at its inlet. In the disclosed combustor, the individual burners are operable in a pilot mode and in a premix mode, wherein within one burner group all the burners are constantly operated either in the premix mode or in the pilot mode. According to the operating mode, the burners require more or less fuel, which is adjustable by the distribution valves. The operation of the distribution valves takes place in the disclosed combustor in dependence upon the respective load state of the combustor.
To achieve emission values for pollutants which are as low as possible, the burners are operated as lean as possible at the nominal operating point of the combustor. By means of the lean operation, the homogenous combustion reaction, which is in process in the combustion chamber, leads to comparatively low temperatures. Since the formation of pollutants, especially the formation of NOx, depends disproportionately on the temperature, the low combustion temperatures lead to a reduction of the pollutant emissions. On the other hand, it has been shown that a homogenous temperature distribution in the combustion chamber promotes the creation of pressure pulsations. Thermoacoustic pressure pulsations, on the one hand, lead to a noise nuisance, and on the other hand, can disadvantageously influence the combustion reaction. In an extreme case, strong pressure pulsations can extinguish the flame in the combustion chamber. In this case, it has been shown that with less lean, or with rich fuel-oxidant mixtures, the combustion reaction is less susceptible to thermoacoustic instabilities. Especially, zones with rich combustion can stabilize adjacent zones with lean combustion.
EP 1 050 713 A1 discloses a method for suppression or control, as the case may be, of thermoacoustic oscillations in a combustor, in which the aforementioned oscillations are detected in a closed control loop, and acoustic oscillations of a defined amplitude and phase are generated in dependence upon the detected oscillations and are coupled into the combustion chamber. By this measure, the thermoacoustic oscillations are suppressed or reduced, as the case may be, if within the control loop the amplitude of the generated acoustic oscillations is selected to be proportional to the amplitude of the detected oscillations. By this method, therefore, the thermoacoustic oscillations which arise in defined operating situations are damped.